Production of grain oriented electrical steels wherein a preconditioning step is employed prior to decarburizing



United States Patent O 3,162,553 PRODUCTION OF GRAIN ORIENTED ELECTRICAL STEELS WHEREZN A PRECGNBITIONING STEP IS EMPLOYED PRiOR T DECARBURIZING Peter N. Richards and Donald Ian Cameron, Merewether, New South Wales, Australia, assignors to John Lysaght (Australia) Limited, Sydney, New South Wales, Australia No Drawing. Filed Aug. 21, 1962, Ser. No. 220,118 Claims priority, application Australia, Aug. 22, 1961, 8,333/ 61 3 Claims. (Cl. 148-112) This invention relates to the production of polycrystalline, magnetically soft, electrical steel sheet or coil of the kind generally known as grain oriented electrical steel. More particularly the invention relates to the decarburization step involved in the production of such steel.

A typical method of producing grain oriented steels according to conventional practices may be briefly described as follows:

(1) Alloy steel containing about 3% of silicon is produced and hot rolled into a strip having a gauge of from 0.125 inch to 0.067 inch. The rolled strip is then cooled, or allowed to cool, and allowed to age to produce a suit able metallurgical state therein. This material is usually known as hot strip and is referred to as such hereinafter. Hot strip may be regarded as a conventional raw material from which grain oriented electrical steel is produced.

(2) The hot strip is cold reduced. This is usually effected in two stages although one or more than two stages may be adopted. In any event the last stage in the cold reduction is generally about a 50% reduction to a final gauge of say 0.013 inch or thereabouts.

When the cold reduction is carried out in more than one stage closely controlled annealing operations are effected between the various reductions; these annealing operations being carried out at a temperature of from 900 to 925 of thereabouts.

(3) The cold reduced strip at final gauge is then subjected to a decarbursing and primary recrystallization treatment by holding it at the temperature in the region of 750 to 850 C. in a furnace having a suitable decarburizing atmosphere.

(4) The material resulting from step 3 above is then subjected to a high temperature annealing during which secondary recrystallization is achieved at a temperature of about 950 C. and final purification is effected at a temperature at a range of from 1100 C. to 1200 C.

In conventional processes the decarburizing is usually efiected as a continuous heat treatment at 800 C. or higher by passing the strip through a furnace containing a moist gas atmosphere. Such continuous decarburization requires equipment which is expensive, particularly if the decarburization is carried out at one of the mentioned intermediate anneals instead of on the thinner material at its final gauge.

The object of the present invention is to provide an improved method of producing grain oriented steel strip or sheet, which is characterized by a decarburizing process which lends itself to the use of relatively simple and inexpensive equipment by comparison with a continuous furnace and which does not require such precise or close control of temperature nor of the composition of the furnace atmosphere, and which does not require the use of as high a temperature as has been necessary hitherto. Contrary to generally accepted practice the dccarburizing step of the present invention is most eco- 3,162,553 Patented Dec. 22, 1964 nomical and etiective when applied to the steel at an intermediate gauge.

The invention consists in a method of making grain oriented electrical steel from hot strip, as defined herein, comprising the steps of cold reducing the hot strip to an intermediate thickness, decarburizing the strip by exposing it to a decarburizing medium while at a decarburizing temperature within the range of from 600 C. to 750 C., removing any surface oxide layers or scale formed on the strip during the decarburizing step, cold reducing the strip to a final thickness and heat treating the strip to effect primary and secondary recrystallization and purification.

According to preferred forms of the invention the decarburizing step is carried out at a temperature of approximately 680 C. and preferably by making use of a solid decarburizing agent such as iron oxide (Fe O or other conventional oxide decarburizers; although a more conventional moist gas decarburizing agent (generally hydro gen or a hydrogen containing gas) may be used.

It has been found, furthermore, that if the strip at the intermediate gauge, that is about twice its final thickness, is raised to a temperature in the region of 700 to 900 C. for a period ot about one to two minutes prior to being decarburized, then the grain oriented steel finally produced is usually of a more uniform quality.

By way of example, a method according to the invention for making grain oriented steel is described below:

Silicon iron containing approximately 3% silicon is produced in conventional manner as hot strip in a suitable metallurgical state.

The hot strip is normalized, generally within the temperature range of from 800 C. to 950 C., and after further cooling is cold reduced to an intermediate gauge about twice the desired final gauge of the grain oriented product.

The strip is then preferably heated to a preconditioning temperature within the range of from 700 C. to 900 C., preferably about 850 C. and preferably continuously in a non-oxidizing atmosphere, it is held at this temperature for say two minutes and after cooling to room temperature is coated with finely powdered iron oxide F6 0, and is then decarburized by being reheated to a temperature of 680 C. or thereabouts and held at that temperature for ten to twenty-five hours.

Next, the oxide is removed by brushing and pickling and the clean strip is cold reduced to the final gauge, cleaned of rolling oils, coated with a suitable separator such as powdered magnesium oxide and finally treated to a high temperature annealing at about 700 C. to effect primary recrystallization. Secondary recrystallization is brought about by further heating to about 950 C. and a final purification treatment is carried out after further heating to a temperature within the range of 1150 to 1200 C. It should be noted that all of the high temperature annealing and purification can be eifected on one furnace as a batch type process because no decarburizing is now involved at this stage. As mentioned above a decarburizing temperature of about 6-8-0 C. is preferred and it is presently believed that this temperature must be Within the range of from 600 C. to 750 C. for the meth- 0d of the invention to be effective.

The invention extends to grain oriented steels when made by the above described novel method.

We claim:

1. A method of making grain oriented electrical steel from hot strip comprising the steps of cold reducing the hot strip to an intermediate thickness, heating said strip fromabout 700 C; to about 900 C., cooling said strip 3. A method according to claim 1 wherein the first to room temperature, and coating said strip with Fe Og; heating of the strip is maintained for a period of about 1 to form a decarburizing medium so as to precondition the to 2 minutes. strip, heating the coated strip Within the range from about V V r 600 C te 750 C. to decarburize the strip, cold reduc- 5 Refefences Cited y the Examine! ing the strip to a final thickness and heat treating the strip r UNITED STATES PATENTS Pnmary secondary recrystalhzatlon and P V Carpenter 2. A method according to claim 1 wherein the strip is heated to about 850 C. during the first heating'step. 10 DAVID RECK Primary Examiner 

1. A METHOD OF MAKING GRAIN ORIENTED ELECTRICAL STEEL FROM HOT STRIP COMPRISING THE STEPS OF COLD REDUCING THE HOT STRIP TO AN INTERMEDIATE THICKNESS, HEATING SAID STRIP FROM ABOUT 700*C. TO ABOUT 900*C., COOLING SAID STRIP TO ROOM TEMPERATURE, AND COATING SAID STRIP WITH FE2O3 TO FORM A DECARBURIZING MEDIUM SO AS TO PRECONDITION THE STRIP, HEATING THE COATED STRIP WITHIN THE RANGE FROM ABOUT 600*C. TO 750*C. TO DECARBURIZE THE STRIP, COLD REDUCING THE STRIP TO A FINAL THICKNESS AND HEAT TREATING THE STRIP TO EFFECT PRIMARY AND SECONDARY RECRYSTALLIZATION AND PURIFICATION. 